1. Field of the Invention
This invention relates generally to photolithography techniques for patterning semiconductor devices, and, more particularly, to a method and apparatus for controlling photolithography parameters based on photoresist images.
2. Description of the Related Art
Conventionally, semiconductor devices are patterned using photolithographic processes. A base material, such as a substrate material, a metal, an insulator, etc., is coated with a light sensitive material, referred to as a photoresist material. The photoresist is generally a composition that is sensitive to active rays of light, such as ultraviolet rays, X-rays or electron rays. The photoresist is deposited on the base material to selectively protect non-process portions of the substrate. Light is then selectively directed onto the photoresist film through a photomask, or reticle, to form photoresist patterns on the base material. The photoresist is then developed to remove either the exposed photoresist or the unexposed photoresist.
There are generally two types of photoresist, namely positive type and negative type. The positive photoresist is of such a type that the exposed portion dissolves in the developer, while the unexposed portion does not dissolve therein, and the negative photoresist is of the opposite type. Certain photoresist materials do not complete the transition from being soluble to being insoluble in the developer based solely on the exposure to light. These photoresist materials, referred to as chemically-amplified photoresists, are subjected to a post exposure bake process to complete the chemical reaction to transition from soluble to insoluble (i.e., for a positive resist).
A known technique for evaluating the acceptability of the photolithography process involves measuring critical dimensions or other parameters after the photoresist has been developed. One method to evaluate the developed wafer is to use scatterometry to generate an intensity measurement indicative of the pattern on the wafer. The pattern in the developed photoresist appears as a series of trenches. Light is reflected differently in the trenched vs. the non-trenched areas, resulting in a characteristic scattering pattern. The scatterometry measurements may be used to change the photoresist operating parameters, such as exposure time, post exposure bake time, develop time, etc. to affect the pattern formed on subsequent lots of wafers. A limitation of a post develop measurement technique is that significant time elapses between the measurement and the corrective action, potentially resulting in numerous unusable wafers.
The process of using a chemically-amplified photoresist is described in greater detail in reference to FIGS. 1A through 1D. FIG. 1A shows a wafer 10 including a base material 12 with a photoresist layer 14 deposited thereon. In FIG. 1B, the photoresist layer 14 is exposed to a light source through a reticle (not shown) to define exposed regions 16. Exposure to the light causes hydrogen free radicals to form in the exposed regions 16. In FIG. 1C, the wafer 10 is subjected to a post exposure bake to complete the solubility transition chemical reaction and form baked regions 18. During the post exposure bake, the free radicals diffuse laterally and react with the photoresist 14 around the exposed regions 16. Typically, for a deep UV photoresist layer 14, the post exposure bake time is about 60-90 seconds. As shown in FIG. 1D, a developer may then be applied to remove the remaining photoresist 14 (i.e., for a negative resist--not shown) or to remove the baked portions 18 (i.e., for a positive resist--shown in FIG. 1D).
The present invention is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.